196 HIROSHI OHSHIMA 
appearance of a syncytium. Mitotic figures are found here 
and, as a result of rapid proliferation, the cells of the distal 
part detach themselves and move into the blastocoele. These 
detached cells continue to multiply after beimg free in the 
blastocoele. While the free mesenchyme cells are amoeboid 
in shape, the dividing ones are readily distinguishable by their 
rounded shape (PIL. 8, fig. 10). 
While rapidly increasing in body length, no mitotic figures 
are found in the ectoderm. The cells here seem simply to 
decrease in height and to extend in surface. It is thickest at 
the hind end near the blastopore, gradually thinning out as it 
approaches the apical end. The nuclei lie near the internal 
surface in the hinder half, while in the apical half they are nearer 
to the outer surface. Only in abnormal embryos, which grow to 
an enormous size without developing beyond the gastrula, were 
many mitotic figures found in the ectoderm. 
When the gastrula reaches its full length the archenteron 
almost exceeds half the length of the whole body continuing 
active cell-division. Selenka (45, p. 164) noticed that the 
archenteron lies in H. tubulosa near the future ventral side, 
and Ludwig (22, p. 605) also found in C. planei that it 
bends slightly ventrad. I have noticed no such feature in the 
ease of C. echinata. At about this stage the archenteron 
begins to flatten in the anterior portion, and then an unequal 
srowth of the wall occurs, resulting in the characteristic twisting 
of the hind part. 
The above-mentioned bundle of long cilia at the bottom of the 
archenteron remains until about this stage. It then seems to 
disappear, and in the same place the archenteric wall now 
begins to bud off cells into the lumen of the archenteron (PI. 8, 
fig. 9, bl). The cells thus liberated into the archenteron, some- 
times called ‘ blood corpuscles ’, vary in amount according to 
different. individuals, in some being tolerably numerous while 
there are none at all in others. 
Archenteron.—The archenteron of the fully developed 
gastrula is very characteristically twisted in the sinistrorse 
direction, and may be described in three parts : the flat expanded 
